Electromechanical force transducers, known as "load cells," have long been used in a variety of load-measuring applications. In load cells, a load is usually applied to a structural member, and strain sensors of various designs are then secured to the structural member. The structural member deforms as a known function of the applied force, and this deformation or strain is measured by the strain sensor. The strain sensor generally provides an electrical output which is thus an indication of the load or force applied to the structural member.
In applications where the shape and other characteristics of the structural member are well defined, conventional strain sensors mounted on the structural member provide highly accurate results. However, in other applications, characteristics of the structural member, such as its shape or deflection characteristics, are not well defined. Under these circumstances, the load-induced strain, even if it can be accurately measured, does not provide an accurate indication of the load carried by the structural member, since the magnitude of the strain for a given force is not known.
One technique for measuring the load applied to a structural member having characteristics that are not well defined is to support the load with a structural member having well-defined characteristics. While this will, of course, provide accurate results, such techniques nevertheless have several inherent disadvantages. First, mounting the load bearing structure on a structural member can often require a great deal of machining to properly connect the structural member to the load-bearing structure. Second, the structural member having well-defined characteristics undesirably increases the size and/or weight of the overall structure. Third, structural members having well-defined characteristics do not often match the structure that they are supporting, so the overall combination can be quite unsightly. Other disadvantages of supporting a load-bearing structure on a structural member having well-defined characteristics will be apparent to one skilled in the art.
In order to minimize the disadvantage of supporting a load with a structural member having well-defined characteristics, attempts have been made to utilize a relatively thin disc as the load-bearing structural member and then measure the load-induced strain in the disc to provide an electrical indication of the load. Such discs have the advantage of being compact, lightweight and relatively inobtrusive. However, discs deflect in a nonlinear manner so that conventional strain gauges providing an electrical output in proportion to the strain produce an electrical output that is a nonlinear function of the applied load. As a result, such load cells cannot be used without either manual or automatic conversion or linearization of the output signal. In the case of manual linearization, in which the load is determined by look-up tables, linearization is time-consuming and error prone. In the case of automatic linearization, the linearizing circuitry can significantly increase the cost of such load cells.